A typical solid-state lithium battery based on solid polymer electrolytes (SPE) consists of a SPE laminate sandwiched between a Li foil anode and a composite cathode laminate. The SPE serves as the separator as well as the medium through which ions are transported between the anode and the cathode. The electrode/electrolyte assembly is usually packaged in a metallized plastic envelope. In some versions of solid-state Li batteries, the anode comprises a compound of Li, such as Li intercalated carbon of the general formula, Li.sub.x C.sub.6, wherein the value of x is usually between 0.1 and 1. The solid polymer electrolyte can be one of several types and include: i) conventional electrolytes such as complexes of Li salts with long chain polymer hosts; for example, LiClO.sub.4 complexes of poly (ethylene oxide), PEO, and ii) non-conventional electrolytes consisting of a Li salt solution, formed in an organic solvent (or a mixture of organic solvents), immobilized in a polymer matrix. Examples of the latter type of solid electrolytes include: a) that composed of 21 mole-percent polyacrylonitrile (PAN), 38 mole-percent ethylene carbonate (EC), 33 mole-percent propylene carbonate (PC) and, 8 mole-percent LiClO.sub.4 ; and related electrolytes (U.S. Pat. No. 5,219,679 and U.S. patent application Ser. No. 08/048,683), and b) that composed of 15 weight-percent (w/o) poly(vinyl chloride), (PVC), 40 w/o PC, 40 w/o EC and 5 w/o LiClO.sub.4 (U.S. Pat. No. 5,252,413). As described in these patent applications Li/LiMn.sub.2 O.sub.4 and C/LiMn.sub.2 O.sub.4 solid-state cells based on these non-conventional polymer electrolytes have been fabricated and cycled (discharged and charged). The Li/LiMn.sub.2 O.sub.4 cells are cycled between the potential limits of 3.5 and 2.0 V and the C/LiMn.sub.2 O.sub.4 cells are cycled between 4.2 and 2.0 V.
Unlike electrochemical cells containing aqueous electrolytes, those containing organic electrolytes, liquid- or polymer-based, may not be overcharged; that is, their potentials during charge may not be allowed to go beyond values where the full electrochemical oxidation of the cathode occurs. This is because overcharge can lead to electrochemical oxidation of the organic solvents and the process almost always is irreversible. Consequently, overcharging of an organic electrolyte-based cell can lead to its catastrophic failure. In the case of the aforementioned Li/LiMn.sub.2 O.sub.4 and C/LiMn.sub.2 O.sub.4 cells, electrolyte is oxidized at potentials .gtoreq.4.3 V versus Li.sup.+ /Li. When these electrochemical cells are cycled in the laboratory, their voltage limits are carefully controlled below these value by electronic cyclers to prevent overcharge. Electronic overcharge control usually comprises a sensing circuit which prevents current from flowing into the cell once the cell voltage reaches the value corresponding to full charge of the cell. However, a drawback of the use of electronic overcharge controllers as a battery component is that it lowers the energy density of the battery and increases its cost.
Overcharge control is also important when single cells are configured to form a battery. In this case, capacity balance among the cells in the battery may be lost, especially after repeated discharge/charge cycles. That is to say that the accessible capacity of individual cells may not remain equal. The reason is the following. When a battery possessing at least one cell with a lower capacity than the others is charged, the cathode potential of that cell will rise above the normal charge voltage limit. If the electrolyte is not stable at the higher potentials, oxidative degradation of the electrolyte will occur, and the cycle life of the battery will degrade at an accelerated rate. Even if the electrolyte does not decompose, the capacity of individual cells in the battery will increasingly get out of balance with each additional cycle. This is because the stronger cells will not be charged to their full capacity since the weaker cell contributes a larger fraction of the charge voltage of the battery. While electronic overcharge control circuits for each individual cell can mitigate the capacity imbalance problem in a battery, such devices will add significantly to the cost of the battery and decrease its energy density.
A better approach for controlling overcharge is to use a chemical redox shuttle. Here, a material with an appropriate oxidation potential is dissolved in the electrolyte. This material is unreactive until the cell is fully charged. Then, at a potential slightly above the normal charge cutoff voltage of the cell, the redox shuttle is electrochemically oxidized to products. The oxidized products diffuse to the anode where they are regenerated either by reduction or by chemical reaction. The reduced species are in turn oxidized at the cathode and thus a fixed potential is maintained at the cathode indefinitely, until charging is terminated. In other words, the cell potential during overcharge will be "locked" at the oxidation potential of the redox shuttle reagent.
Desirable properties of a redox shuttle include good solubility in the electrolyte, an oxidation potential slightly higher than the normal charge limit of the cell but lower than the oxidation potential of the electrolyte, ability to reduce the oxidized form of the reagent at the anode without side reactions, and chemical stability of both the oxidized and reduced forms of the reagent in the cell.